A PLL (phase locked loop) circuit has conventionally been used for a frequency synthesizer and a spectrum analyzer (refer to Patent Document 1: Japanese Laid-Open Patent Publication (Kokai) No. 2000-40959 (FIG. 1, FIG. 4, and ABSTRACT)). The PLL circuit includes a voltage-controlled oscillator (VCO) and a loop filter as is generally known.
In order to quickly lock an oscillation frequency of the voltage-controlled oscillator to a target frequency, a D/A converter is used for pre-tuning (Refer to Patent Document 1). Specifically, data of a voltage to be impressed on the voltage-controlled oscillator so that the voltage-controlled oscillator oscillates at the target frequency is converted into an analog voltage by the D/A converter, is added to the output of a loop filter, and is then impressed on the voltage-controlled oscillator (for example refer to FIG. 4 of Patent Document 1). By means of the pre-tuning, the oscillation frequency can be close to the target frequency. After the pre-tuning, it is possible to quickly lock the oscillation frequency to the target frequency as a result of a normal operation of the PLL circuit.
In order to more quickly lock the oscillation frequency to the target frequency, the gain of the loop filter may be increased.